In recent years, in the field of motor vehicles, a drive assist system to support (assist) a driver's operation by performing vehicle control in correspondence with behavior and surrounding environment of the vehicle, is put into practical use. Among these systems, a map-based ADAS (Advanced Driver Assistance Systems) to perform vehicle control such as control of accelerator, a brake, or steering, based on map information is known. As an actual example of the map-based ADAS, a function referred to as an “intelligent pedal” is known. This function is to look ahead a road map shape ahead of the vehicle in a traveling direction of the vehicle, to predict a curve approaching the vehicle, and push back an accelerator pedal when the speed is excessive, so as to call the driver's attention.
To realize this map-based ADAS, a map matching technology is indispensable. That is, to look ahead road map shape ahead of the vehicle, it is necessary to accurately detect (determine) the position of the vehicle on the road map by map matching processing. The map-based ADAS is actuated on the assumption that the map matching processing is properly performed. As a technique related to the map matching processing, a technique described in Patent Literature 1 is known.